A serial attached SCSI (SAS) storage domain may include a central SAS switch. The domain may also include a redundant pair of SAS RAID controllers attached to the switch with cables and one or more SAS expanders, housing storage devices, also attached to the switch with cables, either directly or in a tree or daisy chain topology through other, upstream SAS expanders. The storage domain may then be partitioned into sub-domains, each comprising, for example, one first tier SAS expander directly attached to the switch with a cable. A sub-domain may also include one or more downstream second tier SAS expanders attached with a cable to the first tier SAS expander and/or one or more third tier SAS expanders attached with a cable to a second tier SAS expander. Thus, the topology of each sub-domain may resemble a tree or daisy chain.
It will be appreciated that the SAS devices in a storage domain (RAID controllers and SAS expanders) may be located at different distances from the SAS switch due to the physical layout of the data facility. Consequently, cables of different lengths are required for connection to the switch and to each other (in a sub-domain). Previously, with transmission of data at relatively low data rates, such as 1.5 Gb/sec., special conditioning may not have been required. However, as data rates continue to rise, such as to 3, 6 and eventually 12 Gb/sec., signal conditioning becomes important, particularly when the distance between devices, and hence cable lengths, increases. For a given cable length; the data rate should not exceed an established rate. Put a different way, for a given data rate, the cable length should not exceed an established length. For example, the length of SAS copper cable at a data rate of 3 Gb/sec. should not exceed 10 meters. And, it is expected that the length of SAS copper cable at a data rate of 6 Gb/sec. should not exceed 5 meters.
Some newer cables include non-volatile storage (NVS) storing cable vital product data (VPD) which can be read by an appropriate interface. Commonly-assigned and co-pending U.S. application Ser. No. 11/381,680, entitled MULTIPLEXING A GROUND SIGNAL ON A HIGH SPEED CABLE INTERFACE TO PROVIDE ACCESS TO CABLE VITAL PRODUCT DATA, filed on May 4, 2006, discloses such an interface.
New apparatus, methods and computer instructions are thus desirable to be able to take full advantage of the information provided by cable persistent data, including cable length, and to be able to adjust signal conditioning parameters accordingly.